This is the graph of a cosine function: it has the repeating hill and valley pattern and is at the top of a hill as it passes through the y-axis. The amplitude of this graph and $\displaystyle{y}={2}{\cos{{3}}}{x}\usepackage{cmbright}\Huge\$ is 2.

This graph has 3 cycles in the interval $\displaystyle{\left[{0},{2}\pi\right]}\usepackage{cmbright}\Huge\$ , or a period of $\displaystyle\frac{{{2}\pi}}{{3}}\usepackage{cmbright}\Huge\$ . The same is true for $\displaystyle{y}={2}{\cos{{3}}}{x}\usepackage{cmbright}\Huge\$ , so the graph and the function match.